In the last decades, under the driving forces of energy, environmental and economic constraints, large efforts have been made with the aim of finding new technologies to replace the well-established Kraft process for the manufacturing of chemical pulp. The traditional Kraft process accounts for most of the chemical pulp production in the world and commands several advantages over alternative processes such advantages including insensitivity to wood quality and superior physical pulp properties.
The Kraft process however has some well-known drawbacks such as a low pulping yield, generation of odorous reduced sulphur compounds and capital investments particularly for the chemicals recovery system.
Soda anthraquinone (soda AQ) pulping is a well-known process alternative to the Kraft process, which offers some simplification of the chemicals recovery process, as there is no requirement for a reducing zone in the recovery furnace. Furthermore the odorous and toxic sulphurous emissions are substantially eliminated by the elimination of sulphide as an active pulping chemical. On the other hand, the replacement of sulphide demands a higher charge of sodium hydroxide to the soda AQ cook in order to compensate for the lost effective alkali from hydrolysis of sodium sulphide in the Kraft chemicals recovery cycle. Consequently, the lime reburning and causticizing plant in the soda AQ mill, for a given effective alkali charge to the cook, have to be from 20 to 50% larger than in a Kraft mill with a corresponding pulping capacity. Therefore, on balance, and also considering the weaker pulps of traditional soda AQ pulps in comparison to Kraft pulps, soda AQ pulping has not met with commercial success and only a few mills in the world are practising the process.
Alkaline puping processes such as the Kraft, soda AQ and alkaline sulphite processes use strong alkali, sodium hydroxide, to provide for the alkalinity of the cook. In the Kraft process a chemical reagent referred to as “white liquor” is used for delignification and added to the digester. Typically, the white liquor is an alkaline aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) containing between about 90-100 grams/litre of NaOH and about 20-40 grams/litre Na2S with minor quantities of inert chemicals such as sodium carbonate, sulphate and thiosulphate. Depending upon the wood species used and the desired end product, white liquor is added to the wood chips in sufficient quantity to provide a total charge of alkali of 15-22% NaOH based on the dried weight of the wood.
Typically, the temperature of the wood/liquor mixture in the digester is maintained at about 145° C. to 170° C. for a total reaction time of about 2-3 hours. When digestion is complete the resulting Kraft wood pulp is separated from the spent liquor (black liquor) comprising used chemicals and dissolved lignin.
Conventionally, the black liquor is burnt in a Kraft recovery race to form a smelt comprising sodium and sulphur chemicals. The smelt is dissolved in an aqueous solution, usually in weak wash, to form green liquor, containing Na2CO3 and Na2S, which is mixed with lime (CaO) to form a turbid mixture containing particles of slaked lime (Ca(OH2). The mixture is recausticized according to the scheme.Ca(OH)2+Na2CO3═2NaOH+CaCO3
The alkalinity of the liquor is thereby restored and fresh Kraft white liquor is obtained for use in the digestion process. The sodium sulphide is not participating in the recausticizing process, although sodium sulphide is contributing significantly to the alkalinity of the white liquor. A number of discrete causticizer vessels are normally used to reduce the risk of lime particles migrating directly out of the system without undergoing reaction. Usually, the reacted mixture is passed to a clarifier which separates it into a liquid phase which is strong in NaOH and which is used in the pulping process, and a phase heavy in solids (mainly CaCO3) which is washed with water to reduce its white liquor content, and then passed to a lime kiln where the solids are calcined to yield fresh CaO. Because of the inefficiency of the conventional recausticizing process, a dead load of unreacted Na2CO3, considered as an inert in alkaline cooks such as Kraft and soda AQ, is carried in the white liquor to the pulping process and hence through the Kraft liquor cycle. The white liquor content of strong alkali, all of NaOH and one half of the Na2S content is called effective alkali.
In soda AQ pulp mills the recaustcizing and lime reburning operation is essentially the same as in the Kraft process except that, for a given charge of effective alkali, even larger equipment capacities are needed for the regeneration of strong alkali as there is no contribution of effective alkali from sodium sulphide hydrolysis.
The caustic and lime reburning operation in pulp mills represent a high investment and operating cost and frequently these units are bottlenecks in mill expansion projects.
Over time there has been a considerable interest in finding says to eliminate the lime reburning and causticizing operation in alkaline processes through so called autocausticizing. The proposed autocausticizing processes are normally based on the use of amphoteric salts to release carbon dioxide directly from sodium carbonate in the Kraft recovery furnace. Strong alkali (NaOH) is then generated directly from the smelt in a dissolving tank. The most promising autocausticizing agents are based on boron. Boron based autocausticizing could potentially supply either part or all of the hydroxide requirements in the Kraft pulping process. Janson initiated the use of bores for autocausticising it the pulp and paper industry in 1976 and a US patent was granted to Janson in 1977, U.S. Pat. No. 4,116,759. A full-scale mill trial on Janson's autocausticizing concept was performed at the Enzo Gutzeit linerboard Kraft mill in Kotka, Finland in 1982. The results were inconclusive and the mill discontinued the use of borates for autocausticization. Due to the high load of boron compounds in the pulping liquor, in accordance with the stoichiometry proposed by Janson, the ionic strength of the borate liquor was much higher than the corresponding Kraft pulping liquor. Increased ionic strength of the cooking liquor is commonly said to have a negative impact on the rate of delignification. Furthermore, the large boron charges significantly increased the inorganic load in the recovery cycle.
In their research, Janson and co-workers concluded that the presence of sulphide in the recovery boiler smelts counteracts the autocausticizing reactions of borates, which would be an obvious drawback in Kraft applications. Moreover, for sulphide containing smelts, the presence of carbon dioxide exacerbated the negative effect of sulphide. (Janson J., Autocausticizing alkali and its use in pulping and bleaching, in Paperi ia Puu—Papper och Trä, No 8, 1979, 495-504.) In the binary smelt system Na2S—B2O3), glass formation has been found to occur and compounds of the structure Na2S—nB2O3 (n=2-4) are formed. Thus any sulphide present in the recovery boiler smelt would bind to borates, which else would be available for autocausticizing reactions. Indeed more recent mill scale borate autocausticizing trials in Kraft mills have indicated lower an expected autocausticizing efficiency, which may, at least partly, be due to the presence of sulphide.
Janson concluded hat, of the different borates, sodium metaborate (NaBO2) was too weakly alkaline to be considered for pulping, but quite possible to use in e.g. oxygen bleaching applications. (Janson, J., Paperi ja Puu supra). In his '759 patent Janson teaches, if the borate in its causticized form is sufficiently alkaline which is the case for secondary sodium borate Na2HBO3, it is useable as delignification chemical. Oxygen bleaching experiments are presented in '759 as examples of the use of the weaker alkali NaH2BO3. Janson as well as other researchers in more recent borate pulping studies indeed treat the sodium metaborate as an inert substance during pulping and after the strong hydroxide is consumed in the borate liquor cooks the boron is present as metaborate in the spent pulping liquor.
Of the borates studied by Janson the strongly alkaline tetra sodium diborate (Na4B2O5), or (Na2HBO3) in aqueous solutions, were selected as the source of alkali and this latter substance was used in pulping experiments. The tetra sodium diborate stoichiometry of Janson suggests the presence of one mole of boron compound (as boron) for every mole of regenerated hydroxide in the pulping liquor. After the digestion process, the borate containing spent pulping liquor comprises dissolved light and bore corresponding to the composition of (NaBO2), sodium metaborate. The spent liquor is burned in a recovery furnace and the tetra sodium borate is formed to complete the autocausticizing cycle of Janson.
Janson also briefly discussed the use of anthraquinone in combination with hydroxide or disodium borate (Na2HBO3) as alkali source. It was, however, concluded that the hydroxide based cooks proceeded considerably faster, especially in the early phase, than the borate based cooks. (Janson, J., Paperi ja Puu, supra).
Further work in the area of autocausticizing were performed by Wandelt and co-workers during the 1990s trying to establish whether borate based autocausticizing pulping liquors were as good as sodium hydroxide based cooking liquors in terms of delignification rate, selectivity of delignification, and the quality of the final pulp. The gravity of work by Wandelt and co-workers were on Kraft applications, in other words for pulping systems comprising sulphide, but data were also reported for soda AQ borate alkali pulping experiments. Disodium borate (Na2HBO3) was used as borate alkali. They concluded that “a very slow delignification rate was obtained for sulphur-free soda AQ borate cooking, where instead of 19.5% NaOH (originating from hydrolysis of the Na2HBO3) on wood, 26.7% NaOH had to be used to achieve kappa number 60 during 90 minutes of digestion at 170° C., and it was practically impossible to get bleachable grade pulp of kappa No. 30. Such a process cannot compete with conventional pulping” (Prihoda S., Wandelt P., Kubes G. J., The effect of borates on Kraft, Kraft AQ and soda-AQ cooking of black spruce, in Paperi ja Puu—Paper and Timber, Vol 78, No 8, 1996 p 456-460.)
In these prior art borate pulping studies the sodium to boron molar ratio in circulating liquors was kept well below 2 and indeed, Janson in U.S. Pat. No. 4,116,759 teaches that it is essential to keep the sodium to boron molar ratio equal to or less than 2 in order to ensure desired autocausticization. Sodium carbonate (Na2CO3), commonly considered as being an inert component in a Kraft pulping liquor will be present in typical recovery boiler smelts and, if autocausticization is not 100% efficient, this compound will also be present in the pulping liquor. Sodium carbonate however was not added to any of the borate pulping liquors used in the above referenced pulping studies.
There are recent indications that a key borate compound formed in a recovery furnace would be trisodiumborate (Na3BO3), rather than the tetrasodium borate (Na4B2O5) as suggested by Janson. This has sparked a new wave of interest in borate-based autocausticizing. Trisodium metaborate will form strong alkali and sodium metaborate upon dissolution in water. The overall stoichiometry suggests that only half a mole of borate is needed to regenerate one mole of hydroxide in the liquor system. Two patents have recently been issued in USA using borates for partial autocaustizing combined with traditional lime causticizing, U.S. Pat. No. 6,294,048 and U.S. Pat. No. 6,348,128. Both these patent are based on the use of lime and conventional causticizing to prepare strongly alkaline pulping liquor.
The phase equilibrium diagram of the binary system Na2O—B2O3 shows the existence of the compound trisodiumborate at molar ratios of sodium to boron over about 3:1. Janson suggested that trisodiumborate would not form in the sodium boron smelts because of the strongly basic character of the B2O5 ion but it has been shown experimentally that at least a portion of trisodium borate is formed by reacting berates in excess sodium carbonate at high temperatures. There is, however, evidence on a poor conversion efficiency of reactants to form trisodiumborate in sodium carbonate-borate smelts for example in the body of U.S. Pat. No. 2,146,093 “Method of producing caustic borate products”. A high reaction temperature, at least 1050° C. is needed to obtain trisodiumborate from the reactants and as high as 50 molar percent of the carbonate reactant is still left unreacted in the smelt (FIG. 3 and appended text to FIG. 3 in U.S. Pat. No. 2,146,093). More recently it has been shown experimentally that the reaction of boric oxide in excess of sodium carbonate yields both trisodiumborate and sodium metaborate.
From experimental data in literature, reaction kinetics of the reaction of borates with sodium carbonate to form trisodiumborate appears to be slow, at least below the melting point of the sodium metaborate at 968° C. Recovery boiler smelt zones are normally operating in the temperature range of 900-1000° C. Any presence of carbon dioxide above the reaction mixture, would further depress decarbonisation reactions. A smelt comprising the reactants sodium carbonate and sodium metaborate, injected by the spent liquor in a recovery furnace operating a smelt zone at around 950° C. will thus contain a substantial portion of unreacted sodium metaborate in addition to higher borates such as disodium borate. Moreover, the endothermic nature of the autocausticizing reactions in the furnace smelts may, at least locally, lower the temperature in the char bed increasing the fraction of unreacted sodium metaborate and sodium carbonate in the smelt.
Sodium metaborate (Na2BO2) is rapidly formed it smelts by reacting borates with sodium is carbonate in molar proportions between sodium and boron above about 1:1 at temperatures above about 950° C. At sodium to boron molar ratios lower than about 1:1, compounds with higher boron content such as 2B2O3×Na2O disodiumtetraborate or commonly, anhydrous borax, will be formed.
The dissolving of sodium borates with high boron content in aqueous liquids does not provide for enough alkalinity to be of interest in alkaline pulping applications. For example borax solutions have a pH ranging from about 9-10 at temperature ranges of interest. Moreover, the dead load of inorganic material will increase linearly with decreased sodium to boron ratio in the circulating liquors with proven negative impact on spent liquor viscosity and recovery boiler load.
From the above cited prior art, discussion and experimental evidence it is thus apparent that a substantial portion of sodium metaborate and sodium carbonate will be present in smelts resulting from combustion of boron containing pulping liquors with sodium to boron molar ratios higher than about 1:1. The content of sodium metaborate in the pulping liquor, obtained after dissolving the sodium and boron containing smelt, would in addition to metaborate already present in the smelt also comprise a portion of sodium metaborate from hydrolysis of any trisodiumborate or tetrasodium metaborate formed in the smelt.
As referred to above, pulping liquors based on sodium metaborate with or without the presence of sodium carbonate have hitherto not been considered appropriate for use in line pulping processes.
In the laboratories of the inventor of the present invention new discoveries have been made relating to sulphur chemicals free pulping and a new process named the NovaCell™ process is being tested in mill scale in central Europe. The new process is partly described in PCT/SE00/00288, published as WO 00/47812. Although WO 00/47812 describes a process with several advantages relative to the traditional Kraft process, the capital and operating costs for causticizing and lime reburning is quite considerable for certain applications and wood raw materials.
The major objective of the present invention is to provide an alkaline process for the manufacturing of pulp from lignocellulosic material wherein alkali metaborate is providing alkalinity and buffering capacity during delignification. At least a portion of the alkali used for delignification is recovered from the chemicals recovery cycle in the mill without prior reactions with lime for generation of strong alkali. Other objectives such as elimination of odorous compounds by replacing sulphide with quinone catalysts will be further described in the detailed description and appended claims.